"source" config combined with pytest-xdist leads to incorrect coverage

Describe the bug
Either source behaves weirdly in combination with xdist, or the documentation around
how source/include work could be improved.

When supplying certain source/include values in particular ways with xdist, you can
either get 0% or 100%.

In my real-life usecase, I actually see 16% coverage reported,
where the last xdist process reports some amount of coverage while everything
else reports 0. But perhaps due to the simpler test-setup i lay out below, it only
seems to result in 0s.

It seems, by description, similar to https://github.com/nedbat/coveragepy/issues/389

To Reproduce
How can we reproduce the problem? Please be specific. Don't link to a failing CI job. Answer the questions below:

1. What version of Python are you using? 3.9.6
2. What version of coverage.py shows the problem? 6.3.2
3. What versions of what packages do you have installed? pytest-xdist==2.5.0

❯ tree
.
├── poetry.lock
├── pyproject.toml
├── src
│  └── foo
│     ├── __init__.py
│     └── meow.py
└── test.py

# pyproject.toml
[tool.poetry]
name = "foo"
version = "0.0.0"
description = ""
authors = []
packages = [
    { include = "foo", from = "src" },
]

[tool.coverage.report]
show_missing = true
skip_covered = true

[tool.coverage.run]
source = ["src"]
parallel = true
branch = true

# meow.py
def meow():
    print("meow")


def meow2():
    print("meow")


def meow3():
    print("meow")


meow()

# test.py
from foo.meow import meow, meow2, meow3


def test_meow():
    meow()


def test_meow2():
    meow2()


def test_meow3():
    meow3

If you run the following, you get the correct result (86% in this case)

coverage run -m py.test test.py && coverage combine && coverage report

If you run the following, you get 0%

coverage run -m py.test -n 3 test.py && coverage combine && coverage report

Perhaps notable other options I tried:

source = ["src/"]  # 0%
source = ["foo"]  # 0%
source = ["src/foo"]  # 0%
source = ["src/foo"]  # 0%
include = ["src"]  # 0%
include = ["src/*"]  # 86% hurray!

So tl;dr the behavior of source seems to be different with/without xdist, perhaps
in combination given the fact that the package is nested within a src/ directory
instead of foo/ being implicitly on the path.

*Expected behavior
I expect the behavior of the source option to react the same regardless of
use of the -n flag from pytest-xdist.

Alternatively, if this is difficult to work around, just that perhaps the docs
be made more clear that one needs to use include with a pattern.

Recreate the issue with the example pyproject.toml and test.py, comparing coverage run -m py.test test.py with coverage run -m py.test -n 3 test.py. First verify how the source and include values affect coverage combine and report in each case. Done means xdist produces the same coverage result as the non-xdist run, or the documentation clearly explains the required configuration.